Program change indicator



Nov. 5, 1946. R. A. LYNN PROGRAM CHANGE INDICATR Filed Feb. l5, 1945 is .Songe ok mmf Q Il..

Patented Nov. 5, 1946 PROGRAM CHANGE INDICATOR Roland A. Lynn, Teaneck, N. J., assignor to Radio Corporation of America, a corporation of Dela- Ware Application February 13, 1943, Serial No. 475,728

(Cl. Z50-17) 9 Claims.

This invention relates to program change indicators and has particularly to do with a method of automatically monitoring the modulated wave of a transmitted radio signal and compa-ring it with the original wave at the point of origin. The invention also has other uses, as will be apparent to those skilled in the art.

In the eld of broadcasting the possibility eX- ists, unfortunately, that the wire channel between a studio and the transmitter might be severed by an act of sabotage, and at the same time an unauthorized program might be substituted to modulate the transmitters carrier. The substitution of such a program, if of a subversive nature, could easily create a panic. Program circuits between studios and associated transmitters might be subject to such acts of sabotage without the knowledge of anyone in the studio. Furthermore, no dependable means are available at the transmitter whereby a substitute program could be detected, especially if it appeared to be innocuous.

Accordingly, it is an object of my invention to provide a radio program monitoring system including a comparison device by which an alarm may be sounded, or a visual indicator may be actuated, Whenever a difference occurs between the modulated waves applied to an outgoing line and such waves brought in for comparison by a radio receiver which is tuned to the associated transmitter.

It is another object of my invention to provide a system of the class described in which an alarm device may be actuated whenever there is a departure from a normal ratio between the volume of the studio output signal and the volume of the incoming monitoring signal from the transmitter. This means, of course, that the alarm will be sounded whenever a pause occurs in one modulated wave without a corresponding pause in the other modulated wave. It means, further, that an increased Volume in one wave without a simultaneous increase in volume in the other wave shall likewise be used to actuate the alarm,

According to another object of my invention, means are provided in combination with the aforementioned comparison device for producing a certain limiting action upon the signal energy component which is derived directly from the studio output circuit and which is applied to the comparison device. The purpose of the limiter is to compensate for a limited modulation which is characteristic of the signal as radiated by the transmitter.

In carrying out my invention, I have provided modulator during a period of low volume output' from the studio. Such one-way alarm systems are to be avoided in a dependable monitoring system such as I have disclosed herein.

One of the Weaknesses of the comparison devices used in the past arose from attempts to control the gain in the ampliiiers by half-wave rectication of the signal components. I have foundthat full-wave rectification is practically essential to a successfully operable device of .this character.

My invention will now be described in more detail, reference being made t0 the accompanying drawing, the sole ligure of which represents in diagrammatic form a preferred circuit arrangement.

The terminals I are connected in any suitable manner to a studio output circuit on which a modulated audio frequency wave is impressed. The energy is applied across transformer 2 to a load impedance 3 having a center-tap 4 which connects with a center-tap on the secondary of transformer 2. The impedance 3 may be used as a Voltage divider, taps from which are connected respectively with the control grids in two discharge tubes 5. These tubes are connected for push-pull ampliiication. Their cathodes are connected through a milliammeter E to a suitable point on a voltage divider l. By this connection, the normal cathode potential is maintained at a suitable value above ground potential, as indicated atv the left end section of the Voltage divider 1.

The amplifier tubes 5 are preferably of the pentode type, for the reason that they possess a variable mu characteristic which is of value in obtaining a wide latitude of control action. The screen grids are connected through resistor 32 to junction I6 on the voltage divider 1, Each suppressor grid is connected to its cathode. The anodes are connected to the primary terminals of a transformer 8. This primary has a centertap to rwhich anode potential is applied from the right hand end of the voltage divider 1. A power supply unit 9 is used to obtain all the necessary potentials for the different electrodes of tubes 5 and for other tubes as Well.

The anodes of tubes 5 are also interconnected by an impedance circuit I at the center of which is a connection with the center-tap on the primary of transformer 8. One arm of this impedance circuit is used as a potentiometer for ywithdrawing a signal voltage component to be used for amplitude limiting. This signal component is impressed through an impedance which includes resistor Il, capacitor I2, and resistor i9, the latter being connected to the voltage divider 1 at point I8. The control grid in a discharge tube I3 responds to this signal potential as applied acrossV resistor I9. This tube is preferably of the duplex-diode-triode type. The triode section includes the cathode, control grid, and main anode I4. The diode section includes two anodes I adjacent the cathode. The D. C. potential of the cathode is fixed by its connection to point I1 on potentiometer 1.

The main anode I4 is supplied with a plus D. C. potential from a point I5 on the potentiometer 1, this point being connected to one terminal of a primary winding in transformer 33. The other terminal of this primary is connected directly to the anode I4.

Transformer 33 has a secondary winding, the terminals of which are connected respectively to the two anodes I5. This secondary also has .a center-tap which is connected to ground through resistor 22. The control grids in the push-pull amplifier tubes 5 are'biased in part by the potential drop across resistor 22.

A D. C. potential for normal grid bias is derived from point I8 on the potentiometer 1 and is impressed upon the control grid of tube i3 through grid leak resistor I9.

From the above description, it will be seen that the triode section of tube I3 is enabled t'o amplify a signal component, and this amplified component is rectified, causing a negative bias potential to be impressed upon the control grids in tubes 5 in dependence upon the amplitude of the signal itself. Thus excessive peaks of energy are limited and only signals within a limited volume range are fed across the transformer 3. The gain reduction is Very rapid. The time constant value of the gain recovery is dependent upon the values of capacitor 2l and the grid leak resistor 22. These values may, for example, be of the order of .25 mfd. and 4 megohms, respectively.

The limiter tube I3 serves a very useful purpose under ordinary circumstances. It operates to rapidly establish a lowered gain characteristic upon the advent of an audio wave form of abnormally high peak value, but does not disturb the established operating bias of the tubes 5 during the presence of signals within a normal range of amplitude variation. The desired degree of the limiting action may be selected by adjustment of the tap on potentiometer I0. In certain cases the limiter tube might be dispensed with and a conventional automatic gain control circuit may be vconnected to the input circuit of the amplifier tubes 5.

The limited modulation wave output from tubes 5 is fed across transformer 8 tothe anodes of a double-diode full-wave rectifier tube 23. A desirable attenuation of the higher audio frequencies is obtained by the use of capacitors 24 whichi are shunt-connected across the anode leads of tube 23. Grounded by-pass condensers 38 may also be provided for the same purpose.

The common cathode of tube 23 is connected to point A on a balanced bridge circuit consisting of four resistors 25, 25, 21, and 28. Resistors and 26 are preferably made equal to each 5 other. Resistors 21 and 28 may, if desired, be of the order of one megohm each. These values, however, are not critical.

Point C on the bridge circuit is connected to the mid-tap on the secondary of transformer 8. Point D is grounded. Point B on the bridge circuit is diametrically opposed to point A and receives rectified signal potentials from a circuit presently to be described.

Referring now to the lower portion of the circuit diagram, I show input leads 4I to which are applied audio frequency output energy from a monitoring radio receiver. The primary winding of a transformer 42 is connected to the terminals lil. The secondary winding 3d of transformer e2 feeds energy to a load impedance 44 which is preferably composed of a potentiometer having a tap connected to the control grid of an amplider tube 45. One terminal of the secondary 34 is grounded and is also connected through a cathode resistor it to the cathode of tube 45. Resistor le is preferably shunted by a capacitor c1.

Although the amplifier tube 45 is shown as a triode, tubes of other types may be used if desired. Anode potential is supplied to the tube 45 through the primary winding of a transformer di! from junction point I6 on the potentiometer 1.

Transformer 4B is used for the purpose of coupling the output from tube 45 to lthe anode cir- 35 cuit of a double-diode full-wave rectifier tube e3. The secondary winding of transformer 48 is center-tapped andV is connected to point C ,on the above described bridge circuit. The terminals of the secondary of the transformer 48 are connected respectively to the anodes in tube 43. The common cathode of tube 43 is connected .to point B on the bridge circuit and also to a control grid in a discharge tube 43 which is used as a control value in a relayv circuit.

rIwo discharge tubes t9 andil are suitably connected to be controlled by differences of potential in different arms of the bridge circuit so that either one of two relays 52 and53 may be cie-energized whenever an unbalance of po- 50 tentials occurs in the bridge circuit. Normally the relays 52 and 53 are energized by a conductive state in the two tubes 49 and 5l). The relays 52 and 53 are disposed in the anode circuits of tubes 43 and 53 respectively. These anode circuits also preferably include milliammeters 5l for convenience in observing the operation of the system.

The power supply unit 9 furnishes current for operation of the relays 52 and 53 through the space paths of tubes 49 and 5@ respectively. The relay circuits are Completed through the cathodes of tubes il@ and 53, through the two arms of a potentiometer `54%, and through series resistors 55 and 55 to ground. Resistor 56 is preferably made adjustable and the tap on potentiometer 5d may also be adjusted for balancing the amplitudes of the currents through tubes 4:9 and 50.

In order to facilitate the starting up or resetting of the apparatus after an alarm and particularly in order to energize relays 52 and 53, it is advantageous to momentarily short-circuit at least a part of the impedance of elements 55 andv 56. This is accomplished by actuating a 7.5 key (not shown) which closes contacts 51 land thereby places resistor 58 of relatively low resistance value in shunt with resistors 55 and 59. 'I'his operation enables the tubes 49 and 50 to draw more than a normal now of current and thus to energize the relays 52 and 53. Under normal operating conditions, the current flow through tubes 49 and 55 is sumcient to hold up the armatures 58 and 19, but may be insufficient to attract them across their gaps.

Any one or more alarm devices may be actuated whenever either of the relays 52 and 53 is sufliciently de-energized by at least partially blocking the associated tube 49 or 59 respectively. The armature 60 of relay 52 is associated with a back contact 6l, the latter being connected to' one terminal of an energy source 63. l The other terminal of this source is connected through a switch 64 to common return circuit 65 for all of the alarm devices.

Illustratively, four different types of alarm device are indicated in the diagram. These are lamps 66 and 16, buzzers 61 and 11, annunciators 68 and 18, and a monitoring speaker 40. The circuits for the different alarm indicators '66, B1, and 68 are preferably in parallel. The indicator units 16, 11, and 18 are also connected in parallel. Although the individual indicator units may be separately cut in and out of service by individual switches, it is a simpler matter to provide one switch 64 for opening them all at once after the alarm has been noted.

Relay 53 has an armature 1B associated with a back contact 1l for controlling the parallel connected circuits through indicator lamp 15, buzzer 11, and annunciator 18. These alarm circuits are likewise actuated whenever the armature 10 is released by relay 53, that is to say, when the tube 50 becomes sufficiently blocked.

The winding of a relay 31 may, if desired, be inserted between the common leads 3B and 65 of the alarm indicator cricuits. Relay 31 has conn tacts which are closeable to short-circuit an attenuating resistor 39 in the voice circuit of a monitoring loud speaker 4l). Thus the relatively low-level input to the speaker 49 may be raised to a high level whenever either of the alarm relays 52, 53 releases.

The operation of my program change indicator is further explained as follows:

The voltage developed across resistor 25 in the bridge circuit is dependent upon the amplitude of the signal component which is rectified by the tube 23. Likewise, the voltage developed across resistor 2S is attributable to the amplitude of the signal component rectified by tube 43. When the voltage drop in resistor 25 equals that in resistor 26, then equal biases are applied to the grids of tubes 49 and 59 and these biases are of such value that emission takes place in both tubes, and they normally draw sunicient current to maintain the relays 52 and 53 in an energized state. Under this balanced condition, regardless of the signal amplitudes, points A and B are both at substantially ground potential.

When the rectmed signal potential in tube 23 exceeds that in tube 43, then the difference between the potential drop across resistor 25 and across resistor 25 is such that point A on the bridge circuit is rendered positive with respect to ground and point B is rendered negative. This is true because the values of resistors 21 and 28 are equal and point D is grounded. Therefore, the ground potential applied to point D is one with reference to which the voltages of points A and B\move equally in opposite directions, the extent of such movement being dependent upon the degree of unbalance between the rectification components from tubes 23 and 43.

Whenever point A is driven sufficiently negative with respect to ground so that the impedance through the space path in tube becomes too great to permit relay 53 to hold up its armature 10, this armature is released and causes the alarm indication to be made in such manner as to show an abnormal output from the transmitter with reference to the studio output. Also when point B on the bridge circuit is rendered suii'- ciently negative with respect to ground, then the impedance of tube 49 becomes too great to allow armature on the relay 52 to be held up. The alarm indication is then made so as to indicate a failure on the transmitter side of the comparison device.

If the alarm circuits which include pilot lamp 1S, buzzer 11, and annunciator 18 are actuated, then it is known that an abnormal condition has occurred in which the output from the transmitter cannot be accounted for as originating at the studio. The reverse condition is indicated by the actuation of pilot lamp 66, buzzer 61, and annunciator 68 when the tube 49 becomes at least partially blocked and relay 52 de-energizes. The amplitude of the output from the loud speaker 40 will be greatly increased -upon setting off either side of the alarm circuit, provided there is any signal at all which the monitoring radio receiver can pick up.

For the purpose of smoothing the rectified potential output from tubes 23 and 43, I preferably place a delay circuit in shunt with resistors 25 and 26. This delay circuit includes capacitors 29 and resistors 30. The junction between the two capacitors is connected to point C on the bridge circuit and also to the anode circuits of the rectil-ler tubes 23 and 43.

It will be seen from the above description that my automatic program change indicator is of value not only in detecting subversive acts which would interfere with the Iproper operation of a broadcasting system, but it is also of value for giving an alarm whenever an abnormal transmission condition occurs. The loss of normal established levels due to partial or complete failure of any of the equipment in the paths between the points of origin and reception of the transmitted signal makes itself evident in the operation of the alarm system. Failure may occur in the wire lines, at the equipment of the transmitter, at the transmitter itself, the transmitter antenna, the propagation space path, or in the radio receiver. Abnormally high levels of signal reception as well as abnormally low levels on a comparative basis will result in giving an alarm. Various alarm devices, not only such as shown, but of other types as well may be placed at any convenient points and those which are parallel connected with each other may be disposed at different points remote from one another, if desired.

Various modifications of my invention Will, doubtless, suggest themselves to those skilled in the art in View of the foregoing description. Such modifications would, however, be comprehended within the spirit and scope of the invention.

I claim:

l. A program change indicator system for use in connection with a broadcast studio output circuit and with a monitoring receiver, said system comprising separate full-wave rectiers, one

.7 being coupled to said studio output circuit and the other being coupled to the output side of said receiver, means including a iour-armbridge circuit for comparing the instantaneousenergy components delivered by said rectiers, one arm of said bridge circuit being in series with the rectier which delivers the studio energy component, a second arm of said bridge circuit being in series with the rectiner which delivers the monitoring receiver energy component, the third and fourth arms being each grounded at one end thereof and being connected at the other ends to the irst and second arms at the respective' junctions of the latter with the cathode electrodes oi said reotiers, conductive circuits interconnecting the anode electrodes of both rectiiiers with the junction between the nrst and second bridge arms, a pair of relays each having its winding in circuit with the space path of an individual discharge tube, means for controlling said tubes in opposite senses in response to unequal potentials developed across the rst and second bridge arms, and indicating means selectively actuable by said relays.

2. In a. program change indicator which compares audio-irequency potentials from two different sources normally possessing the same wave envelope, the method of actuating a relay-controlled alarm upon the reception of unlike wave envelopes from said sources, said method comprising separately amplifying and rectiiying a full-wave signal component from each source, producing two currents of substantially equal value in response to the rectiiication of said signal components so long as they follow the same wave envelope, causing the amplitudes oi said currents to vary in opposite senses in response to the reception of unlike wave envelopes, causing a limiting action to be produced with' respect to the amplication gain factor which is applied to the signal components from at least one of said sources', and causing said alarm to be actuated by a relay circuit closure which takes place when the disparity between said current amplitudes reaches a predetermined value.

3. In a program change indicator which compares audio-frequency potentials from two different sources normally possessing the same wave envelope, the method of actuating a relay-controlled alarm upon the reception of unlike wave envelopes from said. sources, said method cornprising separately amplifying and rectifying a full-wave signal component from each source, producing two voltages oi substantially equal Value in response to the rectication of said signal components so long as they follow the same wave envelope, causing the values of said voltages to vary in opposite senses in response to the reception of unlike wave envelopes, causing a limiting action to be produced with respect to the ampliiication gain factor which is applied to the signal components from at least one of said sources, and causing said alarm to be actuated oy a relay circuit closure which takes place when the disparity between said voltages reaches a predetermined value.

4. An alarm circuit arrangement comprising two full-wave rectiiiers each transformer-coupled to a diierent source of audio frequency wave energy, the two said sources normally possessing the same wave envelope, a first, a second, a third and a fourth impedance constituting the four arms of a Wheatstone bridge, a connection from the cathode electrodes of one said rectifier to the -junction between the iirst and third im'- pedances, a connection from the cathode electrodes of the other said rectifier to the junction between the second and fourth impedances, circuit means connected to the junction between the nrst and second impedances and branched to the anode electrodes of both rectiiiers for producing potentials across the first and second impedances in accordance with the relative amplitudes of the waveenergy from the different sources, a ground connection to the junction between the third and fourth impedances, a pair of electronic tubes each havingV an input circuit connected between its ground-connected cathode and its control-grid and having a load'inits output circuit, a connection from the control grid of one tube to thejunction between the nrst and third impedances, and another connection from the control grid of the other tube to the junction between the second and fourth impedances, whereby the unbalancing oi potentials across the iirst and second impedances varies the output of said tubes in opposite senses. Y

5. A. circuit arrangement in accordance with claim e in which each said load is constituted as'the winding of a relay, and said circuit arrangement includes an alarm indicator operable upon de-energZation of one of said relays, and

a second alarm indicator operable upon deenergization of the other of said relays.

6. A circuit arrangement in accordance with claim 4 in which each said load is constituted as the winding of a relay, and said circuit arrangement includes a plurality of alarm indicators operable upon release of a relay armature, certain of said indicators being individual to each relay and at least one indicator being common to both relays.

7. A circuit arrangement in accordance with claim 4 in which each said load is in circuit with the winding of a common relay andsaid circuit arrangement includes a load speader the voice circuit of which includes an attenuating impedance, and means operable by said relay for shortcircuiting said attenuating impedance thereby to increase the output of sound from said loud speaker. 1

8. In a system for comparing the modulation envelopes in two trains of signals normally having a common source, one of said trains being applied to and derived from a space path of radio propagation and the other or said trains being applied to and derived from an essentially metallic circuit, separate means for amplifying and rectifying the signal energy in each of said trains, a four-arm bridge circuit arranged to be fed with rectied energy components from said separate means, and an alarm circuit responsive to a condition of energy unbalance in corresponding arms of said bridge in the event that the characteristics of said modulation envelopes in the two said trains diiier by a predetermined amount. v

9. A system according to claim 8 and including two indicators operatively connected to said alarm circuit, one of said indicators being responsive to an unbalance of potentials in said bridge circuit which results from an abnormally high energy content of the signal train derived from radio propagation, and the other of said indicators being responsive to an unbalance of potentials in the opposite sense.

ROLAND A. LYNN. 

